1. Field of the Invention
The present invention relates to a control system for an automatic transmission and, more particularly, to a control system for switching engagement and release of frictional engagement elements in a transmission mechanism so as to shift the automatic transmission.
2. Related Art
One type of prior art automatic transmission has a gear train construction which applies one frictional engagement element while simultaneously releasing another frictional engagement element, e.g., brakes or clutches, in shifting between two specific gear stages. In such a gear train, one-way clutches are arranged in parallel with the individual frictional engagement elements to optimize the timing of application/release to thereby avoid a drop in the output shaft torque due to tie-up or engine racing due to under-lap. In the interest of compactness, other transmissions omit the one-clutch. In this case, oil pressure feeding/releasing oil passages for the two frictional engagement elements muse be provided with dedicated valves for controlling the release of the oil pressure from one frictional engagement element and the oil pressure feed to the other in a related manner. Such a control system is exemplified in the prior art by that disclosed in Japanese Patent Laid-Open No. 157168/1993. In this prior art control system, the valve (i.e., 2-3 timing valve) for controlling the oil pressure of the frictional engagement element to be released for a shift from 2nd speed to 3rd speed (abbreviated as "2nd.fwdarw.3rd" as in other shifts) is arranged separately from a valve (i.e., B-3 control valve) for controlling the oil pressure of the same frictional engagement element in a different shift.
However, the above-described prior art design is not always suitable for making the control system sufficiently compact to meet the demands for a multistage, light-weight automatic transmission. While compactness might be improved by integrating the aforementioned two valves to perform the two functions with a single valve, such an integrated structure would create other problems. Specifically, in one shift (e.g., 2nd.fwdarw.3rd) the valve would have its spool actuated to control the first frictional engagement element (i.e., brake B-3) by applying the oil pressure of the applied side frictional engagement element (i.e., the brake B-2) and the released side frictional engagement element (i.e., the brake B-3) in a first direction and by applying the external control signal pressure (i.e., SLU linear solenoid valve pressure) in an opposite, second direction. In another shift, on the other hand, the first frictional engagement element (i.e. the brake B-3) would be controlled by the relation between the external control signal pressure and only the oil pressure of that element, so that the external control signal pressure for ensuring the complete engagement of the element must be higher for the first-mentioned shift (i.e., 2nd.fwdarw.3rd), than for the remaining shifts. However, the oil pressure range in which the valve (i.e., the linear solenoid valve SLU) can accurately output the external control signal pressure, is limited of itself. If, therefore, the output range is widened to cover such a shift, the accuracy of the oil pressure output at the remaining shifting times is reduced. If, however, the control for the shift is to be made within such an output range as can retain accuracy, the engagement of the first frictional engagement element cannot be retained.